Foz do Iguaçu is spread out and the main attractions are not walkable from the city centre, so you will need transport. Public buses are cheap and run to all the main attractions, but services can be slow and infrequent outside peak hours. The tourist bus (lines clearly marked for the falls, Itaipu Dam, and Triple Frontier) is the easiest option for independent travellers, running regularly from early morning.
Taxis and app-based ride services operate in the city and are reasonably priced by Brazilian standards. Renting a car gives you the most flexibility, especially if you want to combine the falls, the dam, and the Triple Frontier in a single day — they lie in very different directions from the city centre.
Crossing into Argentina and Paraguay is easy. Buses to Puerto Iguazú run frequently from the urban transport terminal and cost very little. The Friendship Bridge to Ciudad del Este carries buses, taxis, and motorbike taxis. Both crossings involve passport checks; allow extra time during peak hours when queues can build.